1. Field of the Invention
The present invention relates generally to the capture of gases emitted from furnaces and more particularly to ways and means for increasing the fuel value of off-gas recovered from oxygen blowing steel converters.
2. State of the Art
In a well-known process for making steel, a stream of oxygen gas is contacted with molten ferrous metal contained in a steel converter vessel. The oxygen reacts with carbon contained in the molten metal. The reaction generates an off-gas which comprises large quantities of carbon monoxide, a gas which has value both as a fuel and as a constituent in the manufacture of chemical products. The off-gas emitted from the converter vessel also contains amounts of carbon dioxide and other "impurities" that dilute the concentration of the carbon monoxide.
Because of the valuable carbon monoxide content of the off-gas but also because of its highly toxic and flammable nature, more recent technology captures the hot (2700.degree. F.) off-gas emitted from the converter vessel in a suppressed combustion type system. The off-gas captured by the suppressed combustion system is then cooled and cleansed of dust particles. The cooled, cleaned gases are either combusted and discharged to the atmosphere or recovered for subsequent use.
A typical suppressed combustion system involves several stages. At the beginning of the oxygen blowing process, an off-gas capture device is positioned above the opening at the top of the converter vessel. Shortly after oxygen blowing begins, the carbon content of the molten metal reacts violently with the blown oxygen and "ignition" occurs.
As oxygen blowing in the converter vessel proceeds, the volume and carbon monoxide content of the off-gas increases. At this occurs, an exothermic reaction between the carbon monoxide and the oxygen contained in the outside air entering the capture device causes the temperature of the gases passing through the device to increase substantially. Although the capture device is typically indirectly cooled by liquid or steam, the heat generated by the reaction of carbon monoxide and oxygen and the resultant increased gas volumes would eventually exceed the design capability of the device if the reaction were allowed to proceed unchecked. Thus, the amount of air entering the capture device is controllably reduced. Combustion of carbon monoxide is thus suppressed and the heat release in the device is maintained at a relatively low level.
For the first few minutes after suppressed combustion has begun, the volume of carbon monoxide emitted from the converter vessel remains too low to make its recovery economically attractive. Usually during this stage, the partially-combusted off-gas captured by the capture device is cooled, cleaned and delivered to a flare stack for complete combustion and discharge to the atmosphere.
When the volume of carbon monoxide contained in the off-gas reaches a point at which it is economically attractive to recover the off-gas in an uncombusted state, flow of the captured off-gas is redirected from the flare stack to a gas holder for recovery. During this stage of the off-gas recovery procedure, a minimum amount (approximately 10%, for example) of the stoichiometric amount of air required for complete combustion of the carbon monoxide to carbon dioxide is drawn into the capture device by an induced draft fan. This amount of air is allowed to enter the system to insure that the toxic, inflammable and dust-laden off-gas does not escape and enter the work area surrounding the converter vessel. Thus, during this stage of the off-gas recovery procedure, about 90% of the available carbon monoxide emitted from the converter vessel is recovered for further use.
As the oxygen blowing process comes to an end, i.e., as the carbon content of the molten metal charge in the converter vessel becomes depleted, the carbon monoxide content of the off-gas decreases. When the carbon monoxide content falls below a point at which recovery is economically worthwhile, gas recovery to the holder is terminated and greater quantities of air are again allowed to enter the capture device. As in the first stage of blowing, the oxygen in the air reacts with the hot carbon monoxide contained in the off-gas to form carbon dioxide. The gases leaving the capture device are cooled, cleaned and discharged to the atmosphere through the flare stack.
As stated above, because it is necessary to air-ventilate the capture device, at best only about 90% of the available carbon monoxide is recovered during the recovery stage of an oxygen blowing process. In addition to causing the combustion of 10% of the available carbon monoxide to carbon dioxide, air introduced for ventilation purposes also introduces nitrogen and other gases to the recovered off-gas to further dilute the carbon monoxide content.
Efforts have been made to reduce or eliminate the requirement for air ventilation of the capture device. For example, either carbon dioxide or nitrogen has been utilized to form an inert gas curtain which surrounds the gap between the converter vessel opening and the inlet of the capture device. The carbon dioxide or nitrogen curtain prevents air from entering the capture device through the gap. Ventilation of the device is maintained by drawing a portion of the carbon dioxide or nitrogen from the curtain, through the gap and into the device. While this procedure substantially eliminates premature combustion of carbon monoxide by reaction with air, it introduces additional carbon dioxide or nitrogen into the capture device to further dilute the carbon monoxide content of the recovered off-gas.